Process for production of reinforced composite materials and products thereof

ABSTRACT

Particle reinforced composite material produced by mixing granules of a composite material formed by rapidly solidifying a melt comprising a based light metal matrix and particles of a non-metallic reinforced material with granules of unreinforced host metal matrix, compacting the mixture and applying a shear deformation on said mixture.

The present invention relates to reinforced composite materials and moreparticularly to a process for the provision of composite alloysreinforced by dispersed particles and the product thereof.

It is known that in order to improve the mechanical properties of metalsit is possible to reinforce a metallic matrix with filaments orparticles having high characteristics which are insoluble in the basemetal. Reinforcing an alloy with ceramic particles, whiskers or fibresyields a material combining the most useful properties of both the metaland the ceramics. The nature and amount of the dispersed particlesenable the obtained composite alloys to be adapted to different advancedtechnical requirements changing besides the mechanical also the physicalproperties such as thermal expansion, conductivity, magnetic propertiesetc. of the original alloy.

Such composite alloys can be obtained e.g. by mixing of granulated basemetal and reinforcing particles followed by an extrusion process. Theresulting materials are, however, liable to several defects likeresidual porosity and poor homogenity, and consequently a considerablereduction in ductility characterizes such extrusions is experienced.

Furthermore, this powder metallurgy route of manufacturing composites israther expensive.

Another process, nowadays widely applied for obtaining composite alloys,is based on melting of a base metal and dispersing of particles in ametal matrix in the liquid phase. An intimate mixture of the particlesand the molten metal can be obtained using this process. However, it isdifficult to avoid sedimentation and segregation phenomena so that theresulting cast composite material may exhibit considerable variations inthe desired homogenity, e.g. between the periphery and the interior of acast block. Furthermore, it has been found that in case of some lowductility alloys the addition of ceramic particles does not result inany significant higher strength in gravity cast specimens.

It is of course possible to use whiskers or continuous fibres asreinforcing means in order to achieve appreciable improvements of thecomposite characteristics. However, the production costs will alsoincrease so significantly that this is not a real alternative to choosefor most applications.

It is therefore the object of the present invention to provide a novelcomposite material, particularly a metal or metal alloy, reinforced byparticles insoluble in the metal matrix and dispersed in a mannerresulting in substantially improved characteristics, especially highstrength and good ductility of the composite alloys.

The present invention is embodied in a process for preparing a compositematerial by incorporating particulate non-metallic reinforcement into amolten matrix material followed by a rapid solidification providing anintermediate granulated composite alloy material, mixing of the obtainedcomposite alloy granules with granules of host metal and finallycompaction and extruding of the resulting mixture.

The base metal can, for example, be aluminium, magnesium, copper,nickel, titanium or their alloys. As particulate additions particlesformed of refractory compounds having high elasticity modulus may beused, such as metal oxides, carbides, silicides or nitrides.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood and described in moredetails by means of the following example(s) and by reference to theaccompanying drawings, FIGS. 1-6, where

FIG. 1 illustrates graphically the ultimate strength and yield strengthof the extruded materials with and without reinforcing particles,

FIG. 2 illustrates the tensile properties of the extrusions at room andelevated temperatures,

FIG. 3 shows in a cross-sectional longitudinal view a photo of theextruded reinforced composite alloy material macrostructure(magnification 13,6),

FIG. 4 shows the macrostructure from FIG. 3 at higher magnification(50×),

FIG. 5 illustrates the distribution pattern of the reinforcing particlestaken at the plan perpendicular to the extrusion direction, and

FIG. 6 is a macrostructural longitudinal cross-sectional picture of areference extrusion.

Light metals, especially aluminium/magnesium and their alloys, have alarge potential for substantial improvements in mechanical properties byreinforcing with ceramic particles. Many possible automotiveapplications for aluminium or magnesium alloys such as pistons, pistonpins, connecting rods etc. require higher strength than the commerciallyavailable alloys can satisfy. It was therefore natural to consider apossible particle reinforcement of alloys like standard casting alloy ofthe type AlSi12CuNiMg showing a good strength both at room and elevatedtemperatures. However, no significant improvement of the alloyproperties was achieved in the gravity cast samples reinforced byceramic particles comprising from 10 to 15 volume % of SiC.

During these trials we have surprisingly found that the strength of suchcomposite materials can be greatly enhanced by a suitable secondaryprocessing of the cast composite material.

EXAMPLES

Commercially available silicon carbide particles of average size 12 μmwere added to molten AlSi12CuNiMg alloy and dispersed through the meltusing a modified melt cleaning rotor of the type disclosed in U.S. Pat.No. 4,618,427.

SiC particles were added in an amount of 10-15% to the above alloy. Theresulting composite melts were then cast into tensile specimens andbillets/ingots for further processing of the particulate reinforcedmaterial, namely extrusion of billets to 12 mm diameter test rods andremelting of ingots using a rapid solidification process to providegranules (needles) followed by extrusion of the resulting solidifiedneedles. Tensile testing carried out on more than 100 specimens did notreveal any significant improvement with respect to tensile strength forthe reinforced specimens compared to the original alloy at castcondition and at two different commercial heat treatments.

FIG. 1 displays graphically test results from the following examinationof extruded samples. The value of the ultimate strength (UTS) and theyield strength (YS) are distinguished by different directions of thehatching and where the higher density of the hatching lines denominatesmaterial comprising reinforcing particles (the same distinctions alsoapply for FIG. 2).

The comparison of tensile strength between extruded specimens from castbillets of the above alloy with and without SiC additions shows only amarginal difference (FIG. 1, area A). The same is true for the UTS andYS for extruded specimens of compacted granules from rapidsolidification of the alloy and the reinforced alloy, respectively (FIG.1, area B). The displayed difference in UTS and YS between theextrusions from cast billets (A) and extrusions from rapidly solidifiedgranules (B) is due to the refined microstructure caused by the rapidsolidification process. Apparently, the addition of SiC particles tothis brittle aluminium alloy does not improve the materialcharacteristics.

Then needles of the base alloy (host alloy) AlSi12CuNiMg were mixed withcomposite needles at approximately equal ratio, compacted and finallyextruded as a particle metal matrix composite rods. The appliedextrusion ratio 1:35 is identical with the ratios used in all previousexperiments and the particle content in the resulting mixed needleextrusion was about 8%. All over the same volume fractions of theparticles were maintained.

As disclosed by the diagram in FIG. 1 (area C) a considerableimprovement of the tensile properties is achieved, with average yieldstrength of 260 MPa and average ultimate strength of 340 MPa,respectively. At the same time a good ductility about 4% is maintainedas reflected in the difference between yield strength and ultimatetensile strength.

All the test rods have been exposed to a commercial heat treatmentcomprising holding at 200° C. for a period of 6 hours.

FIG. 2 illustrates graphically the even more excellent properties of theextruded rods at elevated temperatures compared to the properties atroom temperature. While at room temperature the composite extrusions areabout 40% stronger than the unreinforced matrix extrusions, thecomposite extrusions at 200° C. exhibit an increase of about 50% in thetensile strength compared to the unreinforced base alloy.

The temperature exposure of the specimens prior to testing wasrelatively short, 20-30 minutes, but the structure is expected to bestabile due to the preceding heat treatment.

As a matter of fact the composite extrusions have practically the sameyield and tensile strength at 200° C. as the unreinforced alloy at thesame temperature.

Furthermore, besides the improved properties also a much betterextrudability was achieved, the extrusion speed being approximately fourtimes higher compared to extrusion of cast composite billets.

This extraordinary and surprising strengthening effect seems to becaused by a special distribution of the reinforcing particles asillustrated in FIGS. 3-5. Contrary to the hitherto known compositematerials requiring a homogeneous distribution of the reinforcingparticles in the matrix the extrusions resulting from the mixing ofreinforced/unreinforced needles according to the invention exhibit aheterogeneous distribution of the particles characterized byunidirectional arrangement of discontinuous heavily deformed andparticle enriched zones in the metal matrix.

FIG. 3 shows a macrostructure of the extrusion in a verticallongitudinal cross-sectional view, and FIG. 4 is the same macrostructurerevealing more details by higher magnification of the photographicpicture. The pictures show a heterogeneous structure composed ofdiscontinuous heavily deformed particle enriched zones embedded in themetal matrix. The zones are extending parallelly longitudinally throughthe extrusion in the direction of the material flow caused by theapplied solid forming process (extrusion).

This unidirectional arrangement of the discontinuous particle enrichedzones produces a hard and tough material where the metal matrix areasbetween the zones arrest crack propagation. There are no distinctinterfaces between the essentially particle free matrix and the particleenriched zones so that the composite materials according to the presentinvention achieve a perfect bonding of particle enriched deformed zonesto the base metallic material.

FIG. 5 illustrates the unhomogeneous distribution pattern of thereinforcing particles in a vertical cross-section perpendicularly to theextrusion direction. A typical homogeneous distribution of thereinforcing particles resulting from extrusion of particle reinforcedcast billets is shown as a reference in FIG. 6.

While the invention has been described in terms of preferred embodimentsit is apparent that modifications may be made therein without departingfrom the spirit or scope of the invention as set forth in the appendedclaims. Other solid forming processes than the disclosed extrusion canbe applied, e.g. forging, die forging or rolling. Consequently, otherconfigurations of the discontinuous particle enriched zones than theunidirectional arrangement resulting from the extrusion process will beachieved according to the resulting prevailing direction of the materialflow.

Ceramic materials may also be used as the molten matrix, and other typesof reinforcing particles than the disclosed refractory compounds may beused, e.g. carbon particles.

Furthermore, apart from granulation of rapidly solidified melts, amechanical granulation of the particle reinforced composite materialand/or the host matrix material may be applied prior to the mixing andcompacting steps of the process according to the present invention.

The applied host matrix material (alloy) may have the same compositionas the base material matrix of the intermediate granulated compositematerial, as disclosed by the way of example using AlSi12CuNiMg alloy,or two different matrix materials (alloys) can be used in order toachieve the particular properties of the resulting composite material.

The solid forming deformation process can be an extrusion process wherethe amount of the composite granules is in the range of from 15 to 85%,preferably 40-60%, of the total amount of the composite granules and thehost matrix granules.

We claim:
 1. Process for preparing a composite material comprising abase light metal matrix reinforced by dispersed particles to improve themechanical properties of the material, wherein said process comprisesthe steps ofincorporating particulate non-metallic reinforcement into amolten light metal matrix material, rapidly solidifying the melt toprovide granules or needles of composite material, providing granules ofan unreinforced host metal matrix, mixing the granules of the compositematerial and the host material in a predetermined ratio, compacting themixed granules and finally, applying a shear deformation solid formingprocess on the compacted mixture of granules.
 2. The process accordingto claim 1, wherein the host matrix material has substantially the samecomposition as the base matrix of the composite material.
 3. The processaccording to claim 1, wherein the solid forming deformation process isan extrusion process where the amount of the composite granules is inthe range of from 15 to 85% of the total amount of the compositegranules and the host matrix granules.
 4. The process according to claim9, wherein the amount of the composite granules is in the range of from40 to 60%.
 5. The process according to claim 1, wherein the granules areprovided by a rapid solidification of molten materials.
 6. A particlereinforced composite material comprising a base light metal matrixprepared by the process according to any of claims 1, 2, 5 or 4, whereinthe composite material exhibits a heterogeneous macrostructurecomprising discontinuous heavily deformed particle enriched zones in asubstantially particle free matrix.
 7. The composite material accordingto claim 6, wherein the material comprises an aluminium alloy reinforcedby ceramic particles and exhibits up to 50% higher strength than thebase alloy material at a temperature of 200° C.
 8. The compositematerial according to claim 6, wherein the discontinuous particleenriched zones extend unidirectionally.